Ruthenium-catalyzed cycloaddition of 1,6-diynes and nitriles under mild conditions: role of the coordinating group of nitriles.

In the presence of a catalytic amount of [Cp*RuCl(cod)] (Cp*=pentamethylcyclopentadienyl, cod=1,5-cyclooctadiene), 1,6-diynes were allowed to react chemo- and regioselectively with nitriles bearing a coordinating group, such as dicyanides or alpha-halonitriles, at ambient temperature to afford bicyclic pyridines. Careful screening of nitrile components revealed that a C[triple chemical bond]C triple bond or heteroatom substituents, such as methoxy and methylthio groups, proved to act as the coordinating groups, whereas C==C or C==O double bonds and amino groups failed to promote cycloaddition. This suggests that coordinating groups with multiple pi-bonds or lone pairs are essential for the nitrile components.

Cp*RuCl-catalyzed [2 + 2 + 2] cycloadditions of alpha,omega-diynes with electron-deficient carbon-heteroatom multiple bonds leading to heterocycles.

In the presence of a catalytic amount of Cp*RuCl(cod), 1,6-diynes were allowed to react chemo- and regioselectively with electron-deficient nitriles and heterocumulenes at 60-90 degrees C to afford heterocyclic compounds. The mechanism of the ruthenium-catalyzed regioselective formations of bicyclic pyridines and pyridones were analyzed on the basis of density functional calculations. Cyclocotrimerizations of ethyl propiolate with ethyl cyanoformate or propyl isocyanate gave rise to two of the four possible pyridine or pyridone regioisomers.

Synthesis of benzo-fused lactams and lactones via Ru(II)-catalyzed cycloaddition of amide- and ester-tethered alpha,omega-diynes with terminal alkynes: electronic directing effect of internal conjugated carbonyl group.

In the presence of a catalytic amount of Cp*RuCl(cod), 1,6- and 1,7-diynes connected by an amide or an ester tether underwent cycloaddition with terminal alkynes at room temperature to give rise to cycloadducts in 40-93% yields with 63 : 37-83 : 17 regioisomer ratios.